Sadly, my '91 745T does the same thing, but the other way around: cold idle is fine, warm idle is usually at about 650, where it belongs, but can sometimes go up to about 1300. In other words, it is more often than not idling properly, only occasionally goes up to 1300. Has driven me nuts for years.

Is your speedometer working? What error codes is your LH 2.4 fuel injection computer giving you? The only diagnostic code that I get consistently is the lack of data from the speedometer, or 3-1-1, because the speedo does not work. The FAQ does say that the 3-1-1 error code will cause "Idling problems," but since the problem is not consistent with my car, I don't think this is the cause.

One day, then the outside temperature is nice and comfortable, and I have the time and inclination, I'm going to go over every single sensor feeding data to the LH 2.4 computer. In other words, probably never, but . . . I think the problem may be caused by sensor drift in one or more of the 12 to 15 sensors that feed data to the computer.

Also, it may have to do with making sure the throttle body butterfly is properly set for idle operations. Not particularly difficult, but I can't figure out how to clamp off the little air hose feeding the IAC valve. Not without risking shorting vice-grips against the terminals on the back of the alternator.
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Scott Cook - 1991 745T, 1985 RX-7 GSL-SE, 1986 Toyota Tercel (Don't laugh, it is reliable, faithful AND gets 41 mpg!)